Novel Mesoporous and Multilayered Yb/N-Co-Doped CeO2 with Enhanced Oxygen Storage Capacity

A cubic fluorite-type CeO2 with mesoporous multilayered morphology was synthesized by the solvothermal method followed by calcination in air, and its oxygen storage capacity (OSC) was quantified by the amount of O2 consumption per gram of CeO2 based on hydrogen temperature programmed reduction (H2–TPR) measurements. Doping CeO2 with ytterbium (Yb) and nitrogen (N) ions proved to be an effective route to improving its OSC in this work. The OSC of undoped CeO2 was 0.115 mmol O2/g and reached as high as 0.222 mmol O2/g upon the addition of 5 mol.% Yb(NO3)3∙5H2O, further enhanced to 0.274 mmol O2/g with the introduction of 20 mol.% triethanolamine. Both the introductions of Yb cations and N anions into the CeO2 lattice were conducive to the formation of more non-stoichiometric oxygen vacancy (VO) defects and reducible–reoxidizable Cen+ ions. To determine the structure performance relationships, the partial least squares method was employed to construct two linear functions for the doping level vs. lattice parameter and [VO] vs. OSC/SBET.


Introduction
Cerium oxide (ceria, CeO 2 ) and CeO 2 -based binary or multiple composites are typical oxygen storage materials, which have a wide range of applications in VOC abatement, partial alkyne hydrogenation, oxidative dehydrogenation, and water gas shift [1][2][3][4][5][6][7], even in biomedical applications [8]. CeO 2 is appealing because of its unique structure and intrinsic oxygen vacancy (V O ) defect, which can be rapidly formed and eliminated on the surface of CeO 2 , giving the redox cycle of Ce 4+ ⇔ Ce 3+ . This is the source of the oxygen storage capacity (OSC), which could be described by Reaction (1) and could also be written using Kroger and Vink notations as Reaction (2) [9]: Ce 4+ 1−x Ce 3+ Doping CeO 2 with other metallic cations proved to be efficient in enhancing its OSC [10,11]. Inspired by La/N [12] and Sm/N [13] co-doped TiO 2 for enhanced photocatalytic activities, Yb/N doping should be a feasible method for modifying the OSC of CeO 2 in this work. However, no reports had been found on the OSC of cationic and anionic co-doping of CeO 2 . Based on the similarity-intermiscibility theory and the similar ionic radii of the ytterbium cation (Yb 3+ ; 0.98 Å) and the cerium cation (Ce 4+ ; 0.97 Å), the doping of Yb cations into the CeO 2 lattice was feasible, which could be supported by previous reports [14,15]. Nitrogen (N) was a favorite and preferred non-metal dopant due to its relatively small ionization energy and similar ionic radii to oxygen (O). However, besides N-doped TiO 2 [16,17], there have been very limited reports on doping with N anions into the CeO 2 lattice. Since the O 2− anion (1.38 Å) has a larger size than that of the Ce 4+ cation (0.97 Å) in the ionic crystal of CeO 2 , it was more difficult to be substituted by larger N 3− anions (1.46 Å) [18,19]. In our previous work, we successfully synthesized N-doped CeO 2 using ammonium persulfate as an inorganic nitrogen source [20].

Synthesis of Yb-Doped CeO 2
Yb-doped CeO 2 with different molar concentrations of Yb cations was synthesized using the same procedure as controls, but in the absence of TEA. Typically, appropriate amounts of Ce(NO 3 ) 3 ·6H 2 O and Yb(NO 3 ) 3 ·5H 2 O with a total of 4.0 mmol were dissolved in a 30 mL mixed solution of ethylene glycol and distilled water (10 vol.% H 2 O) in a 50 mL Teflon bottle. Then, the bottle was sealed in a stainless-steel autoclave, transferred to an electric oven, and maintained at 200 • C for 24 h. After the autoclave naturally cooled to room temperature, the resulting precipitate was washed with distilled water and ethanol and then dried at 60 • C for 24 h. Finally, the Yb-doped CeO 2 powders were obtained by following calcination at 500 • C for 2 h in air. The as-obtained Yb-doped CeO 2 powders with different molar concentrations of Yb were labeled as 1%Yb-doped CeO 2 , 2%Yb-doped CeO 2 , 3%Yb-doped CeO 2 , 4%Yb-doped CeO 2 , 5%Yb-doped CeO 2 , 6%Yb-doped CeO 2 , and 7%Yb-doped CeO 2 .

Synthesis of Undoped CeO 2
Pure or undoped CeO 2 was synthesized using the same procedure as controls, but in the absence of Yb(NO 3 ) 3 ·5H 2 O and TEA. Typically, 4.0 mmol Ce(NO 3 ) 3 ·6H 2 O was dissolved in a 30 mL mixed solution of ethylene glycol and distilled water (10 vol.% H 2 O) in a 50 mL Teflon bottle. Then, the bottle was sealed in a stainless-steel autoclave, transferred to an electric oven, and maintained at 200 • C for 24 h. After the autoclave naturally cooled to room temperature, the resulting precipitate was washed with distilled water and ethanol and then dried at 60 • C for 24 h. Finally, the pure CeO 2 powders were obtained by following calcination at 500 • C for 2 h in air.

Characterization
The crystallographic phases of samples were characterized by X-ray diffraction (XRD, D/MAX 2200 PC, Rigaku, Japan) with 40 kV tube voltages and 40 mA current. The morphologies of samples were evaluated by field-emission scanning electron microscopy (SEM; JEOL-7500F, Tokyo, Japan) with an acceleration voltage of 5 kV. N 2 adsorption-desorption isotherms were measured using a QuadraSorb SI surface area analyzer (Quantachrome, Boynton Beach, FL, USA), and the BET-specific surface areas were determined using the Brunauer-Emmett-Teller method. The surface composition and binding energy of samples were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, Waltham, MA, USA). The oxygen vacancy defects of samples were characterized using a Raman spectrometer (LabRAM Aramis, Horiba Jobin-Yvon, Paris, France) with a He-Cd laser of 325 nm.

Evaluation of OSC
Hydrogen temperature-programmed reduction (H 2 -TPR) measurements were employed to evaluate the OSC of Yb/N-doped CeO 2 samples, which were performed using a TP-5080 instrument with a thermal conductivity detector (TCD). Briefly, 0.05 g of sample was pre-treated in a 5 vol.%-O 2 /N 2 flow (30 mL/min) at 500 • C for 1 h and was cooled down under this flowing O 2 /N 2 . Then, the sample was purged with high-purity N 2 to remove the excess O 2 on the surface. Finally, a 5 vol.%-H 2 /N 2 flow (30 mL/min) was introduced into the reactor, which was heating to~960 • C (10 • C/min), and the change in H 2 concentration of the outlet gases was monitored online by TCD. Figure 1a shows the XRD patterns of undoped, 5%Yb-doped, 25%N/5%Yb-doped, and 30%N/5%Yb-doped CeO 2 . All identified peaks matched well with the standard CeO 2 (JCPDS no. 34-0349) pattern, indicating that the CeO 2 phase with a fluorite structure was obtained, and no other phases (such as CeN, YbN, and Yb 2 O 3 ) were detected. Moreover, the corresponding lattice parameters were estimated using Bragg's equation; the results and the appropriate linear fitting are shown in Figure 1b. The lattice parameters of Yb-doped CeO 2 and Yb/N-co-doped CeO 2 were greater than those of undoped CeO 2 , which revealed that the possibly partial substitutions of Ce 4+ (0.97 Å) and O 2− (1.38 Å) ions by the larger Yb 3+ (0.98 Å) and N 3− (1.46 Å) ions happened in the CeO 2 lattice, and the local lattice distortion (such as lattice expansion) occurred as a result. For Yb-doped CeO 2 , the lattice parameters increased linearly with increasing Yb content until 5%, suggesting that the concentration of Yb in CeO 2 reached the solid solubility limit with the addition of 5% Yb. Upon the introduction of TEA, the lattice parameters of 5%Yb-doped CeO 2 continued to increase almost linearly until 20%N, implying that N anions were saturated in the 5%Yb-doped CeO 2 lattice with the addition of 20%N. In addition, when Yb and N ions reached the solid solubility limit in CeO 2 , and the amount of Yb(NO 3 ) 3 ·5H 2 O or TEA continued to increase, their lattice parameters decreased, which indicated that the solid solubility limits of Yb and XPS analysis was performed to determine the chemical composition of Yb/N doped CeO2 and identify the influence of Yb/N doping on VO defects and Ce n+ ions in the CeO2 crystal. Figure 2a shows the XPS survey spectrum of 20%N/5%Yb-doped CeO2. All widescan spectra showed clear CeO2 features by the signals ascribed to Ce 3d and 4d and O KLL and 1s. Fortunately, the Yb signal could also be detected by the presence of the Yb 4d SEM was used to study the effect of Yb/N doping on the morphologies of CeO 2 . As seen in Figure 1c, the morphology of undoped CeO 2 was a multilayered structure consisting of flakes, and these flakes intertwined to form an open porous structure. After the addition of Yb and N ions, the morphologies of 5%Yb-doped CeO 2 and 25%N/5%Yb-doped CeO 2 still maintained the multilayered structure, as shown in Figure 1d,e, respectively. Surprisingly, 30%N/5%Yb-doped CeO 2 in Figure 1f has a completely different morphology from the original multilayered structure. It could be concluded that the addition of TEA had a certain effect on the morphology of CeO 2 . Due to TEA's alkalescence, excessive TEA addition would destroy the original equilibrium of the self-assembled multilayer morphology in the solvothermal system, which promoted the formation of spheroid particle aggregates.

Results and Discussion
To further clarify the porous structures of the as-obtained CeO 2 , N 2 adsorptiondesorption experiments were conducted. Figure 1g shows the N 2 adsorption-desorption isotherms of undoped, 5%Yb-doped, 25%N/5%Yb-doped, and 30%N/5%Yb-doped CeO 2 . All isotherms were consistent with type IV hysteresis loops, confirming their mesoporous structure [21]. The specific surface area (m 2 /g) was an objective, reliable, and physically meaningful size metric for porous materials, and the well-known Brunauer-Emmett-Teller (BET) equation was usually used to determine the specific surface area from the physical adsorption of a gas on a solid surface (labeled as S BET ) [22]. The S BET of undoped, 5%Ybdoped, 25%N/5%Yb-doped, and 30%N/5%Yb-doped CeO 2 powders was estimated and is shown in Figure 1h. The S BET of 5%Yb-doped CeO 2 was almost constant with a value of 93.1 m 2 /g, while the S BET value of 25%N/5%Yb co-doped CeO 2 was 107.3 m 2 /g, slightly higher than that of the undoped sample (96.0 m 2 /g), but the S BET value of 30%N/5%Ybdoped CeO 2 was only 52.5 m 2 /g. It could be concluded that the introduction of TEA had a certain effect on the S BET , especially when the TEA addition exceeded a certain value. The morphology not only changed significantly, but the S BET also decreased sharply.
XPS analysis was performed to determine the chemical composition of Yb/N doped CeO 2 and identify the influence of Yb/N doping on V O defects and Ce n+ ions in the CeO 2 crystal. Figure 2a shows the XPS survey spectrum of 20%N/5%Yb-doped CeO 2 . All widescan spectra showed clear CeO 2 features by the signals ascribed to Ce 3d and 4d and O KLL and 1s. Fortunately, the Yb signal could also be detected by the presence of the Yb 4d peak. In addition, the weak peak of the N element could also be clearly observed from the high-resolution XPS spectrum of N 1s in the Figure 2a inset. Figure 2b shows the Ce 3d XPS core-level spectrum of 20%N/5%Yb-doped CeO 2 . The curve of the Ce 3d spectrum was fitted by eight peaks; the bands u 4 , u 3 , and u 1 (and those for v i ) were attributed to the Ce 4+ state, while the u 2 and v 2 bands were due to the Ce 3+ state [23]. Moreover, the O 1s XPS peak of 20%N/5%Yb-doped CeO 2 is shown in Figure 2c, which was divided into four separate peaks by Gaussian distributions, indicative of the presence of four kinds of oxygen species on CeO 2 . The peaks labeled β, γ, and δ could be assigned to lattice oxygen (β for O-Ce 4+ species, γ for O-Ce 3+ species, and δ for O-Yb species), whereas the peak labeled α could be assigned to the chemisorption of oxygen or/and weekly bonded oxygen species related to V O defects [24].
was fitted by eight peaks; the bands u4, u3, and u1 (and those for vi) were attributed to the Ce 4+ state, while the u2 and v2 bands were due to the Ce 3+ state [23]. Moreover, the O 1s XPS peak of 20%N/5%Yb-doped CeO2 is shown in Figure 2c, which was divided into four separate peaks by Gaussian distributions, indicative of the presence of four kinds of oxygen species on CeO2. The peaks labeled β, γ, and δ could be assigned to lattice oxygen (β for O-Ce 4+ species, γ for O-Ce 3+ species, and δ for O-Yb species), whereas the peak labeled α could be assigned to the chemisorption of oxygen or/and weekly bonded oxygen species related to VO defects [24]. The relative concentration of Ce 3+ ions in CeO2, labeled as [Ce 3+ ], could be calculated by comparing the integrated peak areas of the peak related to Ce 3+ ions to those of all peaks in Figure 2b. Meanwhile, the relative oxygen vacancy content in CeO2 (labeled as [VO]) could also be estimated from O 1s XPS in Figure 2c by the ratio of the integrated area of the α peak to that of all peaks. Figure 2d shows   The relative concentration of Ce 3+ ions in CeO 2 , labeled as [Ce 3+ ], could be calculated by comparing the integrated peak areas of the peak related to Ce 3+ ions to those of all peaks in Figure 2b. Meanwhile, the relative oxygen vacancy content in CeO 2 (labeled as [V O ]) could also be estimated from O 1s XPS in Figure 2c by the ratio of the integrated area of the α peak to that of all peaks. Figure 2d shows Raman scattering is a very powerful tool for identifying the nature of surface V O defects [25]. Figure 3a compares the Raman spectra of undoped, 5%Yb-doped, and 20%N/5%Yb-doped CeO 2 . For undoped CeO 2 , the visible Raman spectrum was dominated by the strong F 2g mode of the CeO 2 fluorite phase at~462 cm −1 , as well as a weak band at~592 cm −1 assigned to the defect-induced mode, related to V O defects. This finding indicated that a certain number of intrinsic V O defect sites existed in undoped CeO 2 .
Both the band intensities at~462 and~596 cm −1 had changed with the introduction of Yb and N ions in the CeO 2 lattice. Typically, compared with the defect-induced band of undoped CeO 2 at~592 cm −1 , that of 5%Yb-doped CeO 2 had comparable intensity as the F 2g mode at~462 cm −1 . In the case of 20%N/5%Yb-doped CeO 2 , the band intensity of the defect-induced band was even stronger than that of the F 2g band. Moreover, an alternative approach was provided to estimate the relative concentration of V O defects in CeO 2 , namely, the relative number of V O defects in CeO 2 could be determined by the intensity ratio of the bands at~592 and~462 cm −1 , labeled as I 592 /I 462 . As observed in Figure 3b, with the increasing amount of N in 5%Yb-doped CeO 2 , the relative concentration of V O defects, that is, the value of I 592 /I 462 , increased almost in a straight line and reached a maximum in 20%N/5%Yb-doped CeO 2 . This proved that the additive amount of N anions in fluorite CeO 2 had an important influence on the formation of V O defects in Yb/N-co-doped CeO 2 .
Raman scattering is a very powerful tool for identifying the nature of surface VO defects [25]. Figure 3a compares the Raman spectra of undoped, 5%Yb-doped, and 20%N/5%Yb-doped CeO2. For undoped CeO2, the visible Raman spectrum was dominated by the strong F2g mode of the CeO2 fluorite phase at ~462 cm −1 , as well as a weak band at ~592 cm −1 assigned to the defect-induced mode, related to VO defects. This finding indicated that a certain number of intrinsic VO defect sites existed in undoped CeO2. Both the band intensities at ~462 and ~596 cm −1 had changed with the introduction of Yb and N ions in the CeO2 lattice. Typically, compared with the defect-induced band of undoped CeO2 at ~592 cm −1 , that of 5%Yb-doped CeO2 had comparable intensity as the F2g mode at ~462 cm −1 . In the case of 20%N/5%Yb-doped CeO2, the band intensity of the defect-induced band was even stronger than that of the F2g band. Moreover, an alternative approach was provided to estimate the relative concentration of VO defects in CeO2, namely, the relative number of VO defects in CeO2 could be determined by the intensity ratio of the bands at ~592 and ~462 cm −1 , labeled as I592/I462. As observed in Figure 3b, with the increasing amount of N in 5%Yb-doped CeO2, the relative concentration of VO defects, that is, the value of I592/I462, increased almost in a straight line and reached a maximum in 20%N/5%Yb-doped CeO2. This proved that the additive amount of N anions in fluorite CeO2 had an important influence on the formation of VO defects in Yb/N-co-doped CeO2.  Figure 4a displays the H2-TPR profiles of 5%Yb-doped and 20%N/5%Yb-doped CeO2, as well as undoped CeO2 for comparison. For the H2-TPR spectrum of undoped CeO2, the reduction occurred at 200 °C and reached two maximum H2 consumptions at ~505 and ~776 °C, implying the existence of at least two kinds of oxygen species at various coordination environments. In other words, the reduction behavior of pure CeO2 was divided into two steps: The reduction band at 200~600 °C corresponded to the reduction of surface oxygen species, while the reduction band above 600 °C was assigned to the reduction of bulk oxygen, which could migrate to the CeO2 surface through VO defects and react with H2. Compared to undoped CeO2, the 5%Yb-doped and 20%N/5%Yb-doped CeO2 could release a certain amount of oxygen below 200 °C, and there appeared to be a visible shoulder from 400 °C in the H2-TPR profiles. Moreover, the reduction bands at ~600 °C were far higher than the baseline. These findings indicated that Yb/N doping could effectively improve the OSC of CeO2 by increasing the number of VO defects. In addition, 5%Yb-doped and 20%N/5%Yb-doped CeO2 exhibited a slightly higher redox temperature  Figure 4a displays the H 2 -TPR profiles of 5%Yb-doped and 20%N/5%Yb-doped CeO 2 , as well as undoped CeO 2 for comparison. For the H 2 -TPR spectrum of undoped CeO 2 , the reduction occurred at 200 • C and reached two maximum H 2 consumptions at~505 and~776 • C, implying the existence of at least two kinds of oxygen species at various coordination environments. In other words, the reduction behavior of pure CeO 2 was divided into two steps: The reduction band at 200~600 • C corresponded to the reduction of surface oxygen species, while the reduction band above 600 • C was assigned to the reduction of bulk oxygen, which could migrate to the CeO 2 surface through V O defects and react with H 2 . Compared to undoped CeO 2 , the 5%Yb-doped and 20%N/5%Yb-doped CeO 2 could release a certain amount of oxygen below 200 • C, and there appeared to be a visible shoulder from 400 • C in the H 2 -TPR profiles. Moreover, the reduction bands at~600 • C were far higher than the baseline. These findings indicated that Yb/N doping could effectively improve the OSC of CeO 2 by increasing the number of V O defects. In addition, 5%Ybdoped and 20%N/5%Yb-doped CeO 2 exhibited a slightly higher redox temperature than that of undoped CeO 2 , which was attributed to the substitutions of Ce 4+ and O 2− with the larger Yb 3+ and N 3− and improved the stability of the CeO 2 lattice. than that of undoped CeO2, which was attributed to the substitutions of Ce 4+ and O 2− with the larger Yb 3+ and N 3− and improved the stability of the CeO2 lattice. OSC is a fundamental characteristic and an important indicator of oxygen storage materials. In CeO2-supported catalysts, the porous structure of CeO2 is conducive to the loading of other active components, while the excellent OSC favors regulating the oxygen content of the catalytic system. Therefore, the quantification of OSC was the key to comparing their oxygen storage/release properties. For that, the OSC was quantified by the amount of H2 consumption per gram of CeO2 (mmol H2/g CeO2) based on H2-TPR curves. Finally, the quantified OSC was obtained by calculating the amount of O2 released per gram of sample (mmol O2/g CeO2) according to the H2 consumptions, and the quantified OSC at low temperatures is shown in Figure 4b. Compared with the OSC of undoped CeO2 (0.115 mmol O2/g), that of 5%Yb-doped CeO2 reached as high as 0.222 mmol O2/g with a 93.04% increase. Upon the introduction of N anions, the OSC of Yb/N-co-doped CeO2 continued to increase until 20%N, reached a maximum of 0.274 mmol O2/g with a 138.26% increase, decreasing at higher N content. In our previous work, the OSC of CeO2 increased by 119.02% and 40.22% upon 5% Hf-doping and 3% Sn-doping, respectively [26]. In our previous study, the OSC of N-doped CeO2 microspheres was 0.73 mmol O2/g [20]. Among the existing reported OSC data [27][28][29][30][31][32][33], our OSC in this work was above the average level. Even so, the idea of enhancing the OSC of CeO2 by cation/anion co-doping has been proven to be feasible in this work.
SBET is an important factor affecting the OSC of CeO2, so we developed a function for the OSC/SBET vs. lattice parameter as well as the OSC/SBET vs. VO concentration. These data were linearly fitted based on the partial least squares method, as shown in Figure 5a,b, and the calculated parameters are given as tables in Figure 5a,b. Both Figure 5a,b showed a linear dependence; however, the fitting of the OSC/SBET vs. lattice parameter showed a higher correlation coefficient (R 2 = 0.98257) than that of OSC/SBET vs. VO concentration (R 2 = 0.92349), implying that a larger lattice distortion produced a higher OSC per SBET for Yb/N-co-doped CeO2. This lattice distortion includes not only the VO defects, but also the lattice expansion, as well as the reducible-reoxidizable Ce n+ ions, and so on. Thus, the OSC/SBET could be predicted as a function of the lattice parameter for Yb/N co-doping CeO2. OSC is a fundamental characteristic and an important indicator of oxygen storage materials. In CeO 2 -supported catalysts, the porous structure of CeO 2 is conducive to the loading of other active components, while the excellent OSC favors regulating the oxygen content of the catalytic system. Therefore, the quantification of OSC was the key to comparing their oxygen storage/release properties. For that, the OSC was quantified by the amount of H 2 consumption per gram of CeO 2 (mmol H 2 /g CeO 2 ) based on H 2 -TPR curves. Finally, the quantified OSC was obtained by calculating the amount of O 2 released per gram of sample (mmol O 2 /g CeO 2 ) according to the H 2 consumptions, and the quantified OSC at low temperatures is shown in Figure 4b. Compared with the OSC of undoped CeO 2 (0.115 mmol O 2 /g), that of 5%Yb-doped CeO 2 reached as high as 0.222 mmol O 2 /g with a 93.04% increase. Upon the introduction of N anions, the OSC of Yb/N-co-doped CeO 2 continued to increase until 20%N, reached a maximum of 0.274 mmol O 2 /g with a 138.26% increase, decreasing at higher N content. In our previous work, the OSC of CeO 2 increased by 119.02% and 40.22% upon 5% Hf-doping and 3% Sn-doping, respectively [26]. In our previous study, the OSC of N-doped CeO 2 microspheres was 0.73 mmol O 2 /g [20]. Among the existing reported OSC data [27][28][29][30][31][32][33], our OSC in this work was above the average level. Even so, the idea of enhancing the OSC of CeO 2 by cation/anion co-doping has been proven to be feasible in this work.
S BET is an important factor affecting the OSC of CeO 2 , so we developed a function for the OSC/S BET vs. lattice parameter as well as the OSC/S BET vs. V O concentration. These data were linearly fitted based on the partial least squares method, as shown in Figure 5a,b, and the calculated parameters are given as tables in Figure 5a,b. Both Figure 5a,b showed a linear dependence; however, the fitting of the OSC/S BET vs. lattice parameter showed a higher correlation coefficient (R 2 = 0.98257) than that of OSC/S BET vs. V O concentration (R 2 = 0.92349), implying that a larger lattice distortion produced a higher OSC per S BET for Yb/N-co-doped CeO 2 . This lattice distortion includes not only the V O defects, but also the lattice expansion, as well as the reducible-reoxidizable Ce n+ ions, and so on. Thus, the OSC/S BET could be predicted as a function of the lattice parameter for Yb/N co-doping CeO 2 .

Conclusions
In summary, Yb/N-doped mesoporous CeO 2 with a multilayered structure was synthesized by a solvothermal method using Yb(NO 3 ) 3 ·5H 2 O and triethanolamine as cationic and anionic dopants, respectively. Both the original morphologies and fluorite crystal structure of undoped CeO 2 could be maintained even after the incorporation of 25%N/5%Yb, but the incorporation of Yb/N into CeO 2 could slightly affect their S BET . Both the introduction of Yb cations and N anions could cause lattice expansion, accompanied by the formation of more V O defects and reducible-reoxidizable Ce n+ ions. Compared with the undoped CeO 2 (0.115 mmol O 2 /g), the OSC of 5%Yb-doped CeO 2 increased to 0.222 mmol O 2 /g with a 93.04% increase, while that of 20%N/5%Yb-doped CeO 2 enhanced to 0.274 mmol O 2 /g with a 138.26% increase.